According to JP-2006-77709A (US-2006-0054149A1), a fuel pressure sensor detecting a pressure of a fuel supplied to a fuel injector is used for a fuel injection system which distributes the high-pressed fuel from a common rail (accumulator container) to the fuel injector provided in each cylinder of an internal combustion engine. Besides, the fuel pressure sensor is mounted to the common rail for controlling a pressure in the common rail (rail pressure) so that a detection value of the fuel pressure sensor is equal to a target value. It is diagnosed by the following method whether an abnormality (malfunction) occurs in the fuel pressure sensor.
When the fuel is injected from the fuel injector, the rail pressure descends. Therefore, it is diagnosed that the abnormality (malfunction) occurs in the fuel pressure sensor when a decreasing amount of the detection value of the fuel pressure sensor due to a fuel injection significantly deviates from a specified decreasing amount (standard decreasing amount).
A fuel pressure sensor outputs an output level signal, which is represented by a solid line L1 in FIG. 4, corresponding to a fuel pressure as a detected value. It is likely that the output signal may deviate from the solid line L1 when the fuel pressure sensor deteriorates with age, as shown by solid lines L3 in FIG. 4. In this case which is referred to as an offset abnormality, since a slope of the output signal (solid lines L3) is normal, a decreasing amount of the detected value is not shifted too much with respect to a standard decreasing amount (solid line L1). Thus, even when the above offset abnormality occurs, it is erroneously diagnosed that the output signal is normal, and the above offset abnormality of the fuel pressure sensor cannot be detected.